Chemical Kinetics
Chemical Kinetics
Chemical Kinetics
BY
ERESANYA .O.I
CHEMICAL KINETICS (PART 1)
LECTURE 5
CHEMICAL KINETICS
Chemical kinetics studies the rate at which a chemical process occurs. Apart from information
about the speed at which reactions occur (reaction rate), it also sheds light on the reaction
mechanism (details of all the steps involved in the reaction).
REACTION RATES
Reaction rate can be defined as a change in the concentration of a reactant or a product per unit
time. The unit is moldm-3s-1.
It is a measure of speed of a chemical reaction. There are two extreme types of reactions in
terms of rate:
(a) Slow reactions; These take a long time (hours, days, weeks, months, etc) to complete e.g.
weathering of rocks (breaking down of rocks), formation of coal, rusting of iron and so on.
(b) Instantaneous (fast) reactions; These take a very short time (microseconds or seconds) to
complete. e.g. neutralization reaction, combustion of LPG gas, etc.
Series of experiments have indicated that the rate of a reaction changes with time. The beginning
of a reaction usually has the greatest rate and this rate decreases progressively till the end of the
reaction. It means that a rate must be identified with a specific time.
The measurement of the reaction rate is based on the disappearance of a reactant or
appearance of a product with increasing reaction time.
The rusting of barbed wire indicated below occurs after many years (slow reaction):
Instantaneous reaction
Factors affecting Reaction Rates
The following factors control reaction rates:
(a) Temperature (b) Concentration/ Pressure (c) Presence of Light (d) Catalyst
(e) Nature of Reactants.
(a) Temperature: An increase in temperature typically increases the rate of reaction.
An increase in temperature will raise the average kinetic energy of the reactant
molecules.
Therefore, a greater proportion of molecules will have the minimum energy necessary
for an effective collision.
The figure below indicates Temperature and Reaction Rate.
The relationship between reaction rate and temperature is expressed in the Arrhenius
equation:
Rate = A exp ()
= Ae(- )
Log10 [] x }
x + y = 1 -order reaction
x + y = 2 -order reaction
(c) Presence of Light: Light can speed up some chemical reactions. Such reactions are
called photochemical reactions. It can also alter the cause of others.
For instance, the reaction between hydrogen and chlorine is negligible without light, moderate
in day light and explosive in bright light.
The light activates the reacting particles by breaking their intermolecular bonds and thus
creating active centers. Other examples of photochemical reactions are:
(i) the conversion of silver halide to grey metallic silver, used in photography.
2AgBr + Sunlight 2Ag + Br2
(ii) photosynthesis in plants
(iii) the decomposition of hydrogen peroxide
(d) Catalyst: A catalyst is a substance that accelerates a reaction by participating in it
without being consumed. Catalysts provide an alternate reaction pathway which has
lower activation energy to obtain products.
They are critical to many biochemical reactions. Positive catalyst are used to reduce the
reaction time of many hitherto slow reactions while the negative ones (inhibitors) are
used to suppress unwanted reactions.
Most of the catalysts are specific in their actions , hence, they work on certain reactions
or substances. In addition, most of the catalysts operate well when used in little
quantities. They do not affect the yield of a reaction. Example of catalysts include:
(i) transition metals or their compounds e.g. iron, nichel, palladium, platinum,
manganese (iv) oxide (MnO2) and vanadium(V)oxide (V2O5)
(ii) mineral acids e.g. conc H2SO4, H3PO4 and HCl
(iii)alkalis e.g. KOH, NaOH
(iv) enzymes (biological catalysts) e.g. diastase, maltase, zymase, invertase.
(v) micelles (aggregation of surfactant or detergent molecules in aqueous solution).
The figure below indicates a catalyzed reaction
(e) Nature of Reactants: The nature inform of physical state of matter, bond
type or surface area exposed affects the speed of a reaction. For a substance
in different physical states, the order of reactivity is:
Solid phase < Liquid phase < Gas phase
The solid phase is very slow; the gas phase is very fast and the speed of the
liquid phase is moderate. Many reactions , therefore, are made to take place in
liquid systems. For instance, a liquid reactant is interacted with another liquid
reactant or a solid reactant is first dissolved in a solvent before reacting it with
another reactant.
Also, the type of bonds present in reactants affects reaction rates. For instance,
ionic compounds in aqueous solutions react faster than their covalent
counterparts. This is due to the facts that ionic compounds can produce mobile
ions in solution. The mobile ions can easily acquire energy equal to or greater
than the activation energy of the reaction.
Most reactions require a surface on which reactants come into contact. The
larger the surface area the faster the reaction becomes. For the same masses
of a solid substance (in different forms such as lump, granule and powder), the
smaller the particle size of each form, the larger the surface area exposed to
reaction. Therefore, the order of increasing surface area (which is also the
order of speed of reaction) is:
Lumps < Granules < Powder
ASSIGNMENT
Answer True (T) or False (F) to questions 1-10 below
(1) The reaction rate at the beginning of a reaction is usually the smallest.
(2) Both the presence of a catalyst and temperature affect reaction rates by changing the
amount of activation energy needed.
(3) An increase in concentration is related to the number of collisions inversely.
(4) The reaction speed of 78.9g of iron nails with 120cm3 of 2.5 moldm-3 HCl is the same as
that of 78.9g of iron fillings with 120cm-3 of 2.5 moldm-3 HCl.
(5) The temperature at which a reaction is carried out proportionally affects the number of
reacting particles that acquire the activation energy of the reaction.
(6) The presence of a catalyst affects the pathway of a reaction.
(7) The rate law of a reaction is a mathematical expression showing the relationship
between the reaction rate and the concentrations of the reactants.
(8) Possession of adequate energy is the only condition for obtaining effective collisions
between reactant particles.
(9) The overall order of a reaction can be deduced from the balanced equation of the
reaction.
(10) Activation energy is the energy needed by particles of reactants for effective collisions.
Temperature (0C) 30 40 50
Time (seconds) 80 40 20
(19) The rate of a reaction is proportional to the number of effective collisions occurring per second
between the reactants. This statement is associated with the (a) kinetic theory (b) rate law (c)
atomic theory (d) collision theory (e) gas laws
(20) Which of the following statements is not true about the reaction between dilute hydrochloric acid
and calcium trioxocarbonate(IV)? (a) It is faster after 3 seconds than it is faster after 10 seconds (b)
It slows down with time (c) It eventually stops (d) It proceeds at a constant rate (e) It causes a
loss of mass in reaction vessel contents.
(22) Activation energy of backward reaction is (a) E2 (b) E1 + E2 (c) E1 (d) E2-E1 (e) (E2)3
(23) Activation energy of forward reaction is (a) E1 x E2 (b) E2 (c) E1+E2 (d) E1 (e) E1/E2
(24) Mark the incorrect statements (a) Catalyst provides an alternative pathway to a reaction
mechanism (b) catalyst raises the activation energy (c) Catalyst lower the activation energy (d)
Catalyst alters enthalpy change of the reaction
(25) Write the rate equation for the reaction 2A + B C if the order of the reaction is
zero. (a) R = k[A] 0[B]0(b) R = k[A]1[B]2 (c) R = k[A]1[B]1 (d) R = k[A]3[B]
(26) In the presence of a catalyst, the heat evolved or absorbed during the reaction
--------- (a) increases (b) decreases (c) remains unchanged (d) may increase or
decrease
(27) The rate of reaction that does not involve gases is independent on
(a) temperature (b) pressure (c) catalyst (d) surface area (e) concentration
(28) The ratio of change in concentration of reactant or product to the time taken to
effect the change is (a) reaction rate (b) rate constant (c) concentration (d) gas
constant
(30) The reactions that can be activated by light are called -------- reactions (a) silver
bromide
(b) Photosynthesis (c) exothermic (d) photochemical (e) endothermic
Theory
(1)